1. Field of the Invention
The invention relates to microphones, and more particularly to analog-to-digital conversion of microphone circuits.
2. Description of the Related Art
A microphone circuit converts sound pressure to an electric signal. The electric signal generated by the microphone circuit may be analog or digital. Due to popularity of digital processors, microphone circuits are required to generate electric signals of digital format to facilitate digital processing. Because a sensor of a microphone circuit directly converts sound pressure to an analog voltage signal, the analog voltage signal must therefore be converted from analog to digital as an output of the microphone circuit. Thus, an analog-to-digital converter is a requisite component of a microphone circuit.
Referring to FIG. 1, a block diagram of a conventional microphone module 100 is shown. The microphone module 100 comprises a microphone circuit 110 and a host 120. The microphone circuit 110 converts a sound signal to a digital electric signal D and delivers the digital signal D to the host 120. In one embodiment, the host 120 is a digital signal processor (DSP). The microphone circuit 110 comprises a sensor 102, a gain stage 104, and an analog-to-digital converter 106. The sensor 102 converts sound pressure to an analog electric signal S1. The gain stage 104 then amplifies the analog signal S1 to obtain an analog signal S2 with amplitude suitable for processing in an analog-to-digital converter 106. The analog-to-digital converter 106 then converts the analog signal S2 to the digital signal D as the output of the microphone module 110. The host 120 provides the analog-to-digital converter 106 with a clock signal CLK for analog-to-digital conversion.
For good quality of the digital signal D, the signal-to-noise ratio of the digital signal D must be high enough. An analog-to-digital converter with a high signal-to-noise ratio, however, requires large power consumption. When an analog-to-digital converter 106 with a lower signal-to-noise ratio and thus less power consumption is adopted, a gain value of the gain stage 104 must be carefully determined to ensure the digital output signal D a good signal-to-noise ratio. If the amplitude of the analog signal S1 is small, the gain stage 104 requires a large gain value to increase the amplitude of the amplified analog signal S2 as an input of the ADC 106. If the amplitude of the analog signal S1 is large, the gain stage 104 requires a small gain value to prevent the ADC 106 from saturation.
The gain of the conventional gain stage 104, however, is kept constant and cannot be determined according to the amplitude of the analog signal S1. If the gain stage 104 automatically adjusts the amplitude of the analog signal S2, the host 120 requires information about the gain value of the gain stage 104 for signal processing such as echo cancellation. The data interface between the microphone circuit 110 and the host 120, however, has no path for transmitting information about the gain value of the gain stage 104. The gain of the conventional gain stage 104 is therefore kept constant. When the gain of the gain stage 104 is kept constant, the amplitude of the input signal S2 of the analog-to-digital converter 106 can not be properly adjusted to ensure the digital output signal D a good signal-to-noise ratio. Thus, a method for analog-to-digital conversion in a microphone circuit is required.